In the early 1970's, several researchers proposed so-called liquid crystal "storage" displays in which an uninterrupted layer of chiral nematic liquid crystal was confined between substrates. Examples of such displays were proposed in U.S. Pat. No. 3,703,331 to Goldmacher et al., issued Nov. 21, 1972, and U.S. Pat. No. 3,821,720 to Greubel et al., issued Jun. 28, 1974. Both patents proposed switching the liquid crystal into, and out of, light scattering states by means of applied electric fields. According to these references, this provided a display which could be switched between stable light scattering and light transmissive states.
Though the light scattering phases in these devices were described as "stable" and did in fact persist after the removal of the switching field, the material would eventually relax into the light transmissive state. Consequently, such devices were not truly bistable in that the contrast between the light scattering and light transmissive portions of the display would decay with time and the display had to be refreshed if required to display information for a significant period of time. This is a disadvantage for applications such as signs for use in hotels, supermarkets or elsewhere, in which the display is expected to retain an image beyond the relaxation time of "storage" displays.
Another drawback to the use of these "storage" displays was that the electrical signal required to switch the material from the light transmissive to the light scattering state differed from the signal required to switch the material back from the scattering to the transmissive state, thereby requiring an electronic control capable of generating two different signals. In the display proposed by Goldmacher et al., the chiral nematic liquid crystal had negative dielectric anisotropy and the signal required to switch the material in the light scattering state differed from the signal required to reset the material to the light transmissive state by frequency rather than by voltage. While the display proposed by Greubel et al. used chiral nematic liquid crystal having positive dielectric anisotropy, the reference did not suggest any technique for modulating different portions of the display to form images.
There is a long felt need in the art for a truly bistable liquid crystal display having an energy-efficient switching system. Since the contrast of prior displays degraded or simply disappeared once the switching field was removed, image-forming displays required either a continuously applied electric field or a so-called "active matrix" capable of automatically refreshing the display at intervals short enough to maintain the desired contrast. On the other hand, the most efficient switching system for a truly bistable material would be a passive system capable of applying switching pulses to various portions of the display only when required to define or alter the image.